1. Field of the Invention
The present invention relates to an atmospheric burner. More particularly, the invention relates to an atmospheric burner for use in gas-fueled appliances which reduces emissions of NO, NO.sub.2 and carbon monoxide.
2. Description of the Related Art
Atmospheric burners are commonly used in appliances which use a gas as a fuel. Gaseous hydrocarbons such as methane (natural gas) or propane (bottled gas) may be used as a fuel in such appliances. Common appliances of these types include water heaters, furnaces, boilers, space heaters, ranges and clothes dryers.
When fuels such as hydrocarbon gases are burned in air and the flame temperature exceeds certain levels, it is known that nitrogen present in the air will combine with oxygen to form oxides of nitrogen such as nitrogen monoxide (NO) and nitrogen dioxide (NO.sub.2). These oxides of nitrogen are usually formed simultaneously in the mixed combustion emission gases and are referred to collectively as NO.sub.x, or "nitrogen oxides". Nitrogen oxides are air pollutants, and government regulations set strict limits on the maximum nitrogen oxide emissions from appliances. Consequently, considerable effort has been expended in the area of burner design with the goal of lowering NO.sub.x emissions.
Several approaches for lowering NO.sub.x emissions have been tried, but all to date have required expensive modifications to burner design. U.S. Pat. No. 4,904,179 to Drago and Ripka discloses a burner where a perforated metal screen is disposed in the flame to conduct heat from the flame in an attempt to lower NO.sub.x emissions. This approach disadvantageously requires the screen as a separate component of the burner assembly as well as some provision for mounting of the screen in an appropriate relationship to the rest of the burner assembly.
A gas burner using metal flame inserts in the form of slotted, inverted channel shaped members is disclosed in U.S. Pat. No. 4,616,994 to Tomlinson. The flame inserts of the Tomlinson disclosure are disposed on each side of the gas flames of the burner in contact with the inner cones of the burner flames. This burner design requires a separate assembly from the burner itself, the assembly then being mounted on the burner. The shape of these inserts is complicated and requires a number of manufacturing steps to fabricate.
A different system of flame inserts is disclosed in U.S. Pat. No. 4,652,236 to Viessmann. Flat vertical parallel fins are disposed between slot-shaped gas ports and extend perpendicularly above the gas burner. This arrangement places the fins in contact with the flames to draw heat from the flames. A chimney effect is also said to be produced with secondary air entering the flames and cooling the insert fins. The fins of this invention are disclosed as being separate pieces of material attached to a burner tube. The manufacture of this type of burner is complicated by the need to fabricate these fins separately and then attach them to the burner tube. This approach also reduces efficiency by removing energy intended for heating an external item.
U.S. Pat. No. 4,525,141 to DeWerth et al. describes flame inserts which are rod-shaped rather than flat. The rod-shaped flame inserts are arranged radially around a gas burner and spaced between the flames of the operating burner. Each flame is flanked by two inserts with the insert rods sized so as to have these rods in contact with the inner cones of the flames, which is said to lower the flame temperature by carrying heat away from the flame and radiating this heat. Although the burner allegedly reduces emissions of nitrogen oxides, the design disadvantageously requires more components for its manufacture than most conventional burners. The assembly of the additional components into the finished burner also requires a more complicated manufacturing process resulting in very high production costs.
U.S. Pat. No. 4,629,415 to DeWerth et al. discloses the use of rods formed into concentric rings supported around a circular gas burner. The rods are spaced from the burner ports and arranged vertically so that the gas flames project between the ring shaped rods. The ring-shaped rods of this burner design require additional component parts beyond the conventional burner housing: supports for the rods are needed to hold the rods in the proper position in relation to the burner ports so that the flames will properly contact the rods. These additional components and associated supporting structures increase the production costs associated with this burner.
Secondary air shields have also been used to control nitrogen oxide emissions from burners. Secondary air shields are believed to work by limiting the rate at which air needed for combustion diffuses into a flame. In conventional burners, primary air (air which is mixed with a fuel gas prior to combustion) is normally maintained below the stoichiometric amount required for complete combustion of the fuel gas to prevent detonation. Once ignited, the gas-primary air mixture burns in a flame, but additional air (secondary air) is required for complete burning of the fuel. Secondary air diffuses into a flame from the surrounding space to supply the balance of oxygen needed for complete stoichiometric combustion. By placing a barrier or shield in proximity to a flame, diffusion of secondary air into the flame is slowed. This is believed to affect the fuel gas burning rate in the flame in a manner which reduces emissions of nitrogen oxides.
A conventional use of secondary air shields can be seen in the burner disclosed in Minimizing Emissions of Nitrogen Oxides from Domestic Water Heaters, technical report ES66052D of the Southern California Gas Company, March, 1973, by W. S. Zawada. This report discloses the advantages of a gas burner provided with a solid annular air shield. disposed below the flames of a burner. The air shield is shown as an additional member affixed to the burner assembly, thereby adding to the complexity of the burner design and to its manufacturing cost. Further, the accompanying increase in size renders the burner impractical as a replacement in existing gas appliances.
Thus, known burners have not satisfied the long-felt need in the art for a burner which is inexpensive to produce and efficiently burns gaseous hydrocarbon fuel while producing lower nitrogen oxide emissions.